Neuronal microdysgenesis and acquired lesions of the hippocampal formationconnected with seizure activities in Ihara epileptic rat

The present study was designed to examine the morphological features of the hippocampal formation in the Ihara epileptic rat (IER), and to characteriz e genetically programmed lesion!, and acquired lesions connected with seizu re activities. Neuropathological investigation of the hippocampal formation was performed in four separate groups. 2-month-old IERs with neither abnor mal behaviors nor any seizure activity, and 12-month-old IERs of both sexes with abnormal behaviors, circling seizures or generalized tonic-clonic con vulsions. In every IER examined, there were invariable and fundamental neur opathological findings consisting of abnormal neuronal clusters in the CA1 of the hippocampal formation. Moreover. disarrangement of neuronal cells, s uch as dispersion and gaps in lamination of pyramidal neurons, were observe d. These changes were thought to represent genetically programmed lesions. neuronal microdysgenesis, because they were common findings in 2-month-old and 12-month-old IERs of both sexes. An enlargement of the dentate gyrus wa s also found in rats that experienced generalized tonic-clonic convulsions or circling seizures. This enlargement of the dentate gyms, on the other ha nd, was categorized as a secondary and acquired lesion connected with seizu re activities. It is suggested that the neuronal microdysgenesis in the hip pocampal formation of IER has an intimate relationship with epileptogenesis and/or an enhancement of seizure susceptibility. (C) 2001 Elsevier Science B.V. All rights reserved.